1. Field of the Invention
The present invention relates to a touch probe, used on a coordinate positioning machine such as a machine tool or coordinate measuring machine to enable the inspection of, for example, the dimensions of machined components, and to a signal processing circuit which acts as an interface between the probe and the control of the machine on which the probe is used. Coordinate positioning machines typically comprise an arm to which the probe is mounted, and which is movable relative to a table on which a component or workpiece to be inspected is supported. The machine includes one or more transducers which measure displacement of the arm from a reference position which is usually fixed relative to the table.
2. Description of Related Art
A known probe includes a fixed structure such as a housing by which the probe is mounted on the arm of the machine, and a stylus supporting member supported relative to the housing in a repeatable rest position, from which the supporting member may be displaced when a deflecting force is applied thereto, and to which it may return when the deflecting force has been removed. Measurements are taken by operating the machine to move the arm until a stylus connected to the supporting member comes into contact with the surface of the part to be inspected, and, upon detecting such contact, taking readings from the transducers of the machine to determine the position of the movable arm relative to the reference position. The probe includes one or more analogue sensors which emit signals indicating contact between the stylus and the surface whose position is to be measured. These sensors may sense displacement of the stylus supporting member relative to the housing, or, in high accuracy probes, the sensors may sense deformation of the stylus and/or the stylus supporting member which takes place before such a displacement occurs. Probes employing displacement or deformation sensors are shown in U.S. Pat. No. 4,153,998 or U.S. Pat. No. 4,177,568 respectively.
Signal processing circuitry, which acts as an interface between the probe and the machine control, emits a step-change, or "trigger" signal when the analogue signal from the sensor inside the probe has attained a predetermined threshold set in the interface. The trigger signal instructs the machine control to determine the position of the movable arm and arrest motion of the arm in order to prevent damage to the machine. A small scale movement of the arm relative to the part to be inspected after contact between the stylus and the surface has occurred, known as "overtravel", is accommodated by the ability of the stylus supporting member to deflect relative to the housing of the probe.